Wow, this is quite stunning. I really like the contrast between the chill of the ominous fog in the background and the blazing heat of the magma pouring from the monster's body, then the giant's smoke and the fog meeting until what's hot and what's cold become completely indeterminable. That grey, vanishing skyline really makes the lava stand out.

You really made the what's going through those soldiers' minds really readable for them hardly having visible faces. The shock and apprehension of being greeted by this colossus is quite apparent, as is the determined dedication to defend their city against all odds, which is especially evident in those archers. You're quite nice with subtle body language.

Lol well he looks a lot like some of those earth giants from the game xD Just go to Mount Hyjal, Over in Sethra's Roost... I did so many of those dailies.. They're in other places too, but I remember them there specifically

I´m feeling the unstoppable urge to select the soldier with the mouse, and move them away very slowly, while i click on the barracks (very slowly too) to order them to create more soldiers even if it costs all the gold in stock!

A lot of great things, though everything looks soo stiff, it's way to static, I think you could have chosen more interesting poses for the Giant and the guard with the spear. Also one of the trees in the bg could have been a little diagonally towards the giant to help the focus point

If you're open to some constructive criticism - here's some for you First of all, I like how you use color, with mostly very desaturated colors. Also, how you let the background really recede with lighter tones and lower contrast towards the horizon adds to the depth, again, very well done. What bugs me the most isn't petty stuff like textures or anything, but the poses of the giant and the militia infront of him. They all seem to be pretty static, so you might want to work on more dynamic poses. Especially the guy with the spear looks way too static. The giant for example could have raised one leg, kind of indicating that he's about to stomp the poor guys infront of him. Oh and his hands look a little bit flat, and there's a tangent where his left arm and the tree in the background meet, you might wanna avoid them

As a final suggestion, a view from the bottom up could have helped emphasizing the scale of the giant, since you would make the viewer look up at it, from the militia's point of view. This one would of course mean a major alteration of the whole composition and is merely based on personal taste